(1) Field of the Invention
This invention relates to a compact phosphoric acid fuel cell system whose operation is started and stopped repeatedly, and an operating method thereof.
(2) Description of the Related Art
Conventional phosphoric acid fuel cells are generally operated in the temperature range of 150.degree.-210.degree. C., in consideration of the performance of the cells themselves, durability and life of materials used for them and other requirements. Most of these phosphoric acid fuel cells are large-sized like ones used in electric power plants, and are operated continuously for as a long period of time as thousands hours.
On the other hand, as electric power supply for communication equipment, building or civil engineering works, a compact and portable phosphoric acid fuel cell has recently been drawing attention which can be operated for a desired period of time and at desired occasions, whose operation is started and stopped far more times than those of the large-sized fuel cells.
The above-mentioned conventional large-sized phosphoric acid fuel cells have the following problem.
The phosphoric acid electrolyte of such a fuel cell has a very high concentration when the cell is operated in a high temperature of 150.degree.-210.degree. C. as shown in FIG. 5. When the operation of the cell is stopped with such a high concentration of the electrolyte, the liquid electrolyte turns to be completely solid as a result of icing of the phosphoric acid at room temperature as shown in FIG. 6. This causes electrolytic resistance to grow greatly, making the operation of the cell difficult to start up at room temperature.
In order to change the solidified phosphoric acid electrolyte with high resistance into a liquid one with low resistance again, the cell has to be heated to a certain degree. However, the large-sized fuel cell, for raising its temperature, demands another apparatus to supply a heating medium with great energy because the fuel cell has a large heat capacity itself.
On the other hand, in order to prevent a liquid phosphoric acid electrolyte from solidifying completely, an electric heater has been provided to keep the temperature of the cell over the icing point of phosphoric acid (at 100.degree. C., for example).
However, the compact fuel cell can not employ the above-mentioned heating medium and electric heater because the apparatuses demand an energy supply source and makes the fuel cell system larger and heaviers, which is far from being compact and portable. Therefore, the above problem caused by complete solidification of phosphoric acid must be solved with a different method to realize a compact power source.
One method is disclosed in Japanese Patent Publication No. 3-15305 according to which solidification of phosphoric acid is avoided by lowering the concentration of phosphoric acid electrolyte for the purpose of lowering its icing point. This is executed by supplying the fuel cell with moist gas from at least one side of an oxidizing gas system and a fuel gas system when the operation of the cell is stopped.
However, this method still can not solve the above problem because the fuel cell demands an apparatus to supply moist gas, which makes the fuel cell system larger and heavier without realizing a compact and portable cell.